1. Field of the Invention
The invention is based on a gate-turn-off semiconductor component.
2. Discussion of Background
In U.S. Pat. No. 5,248,622, a gate-turn-off alloyed semiconductor component in the technology of the free pressure contact is specified, the semiconductor body of which exhibits at the edge a silicone resin which acts as a thermal relief when the semiconductor body is built into a housing. Such thermal edge relief is insufficient for non-alloyed power semiconductors.
From EP-A2-0 387 721, a gate-turn-off thyristor is known in which a turn-off electrode at the edge for extracting the minority charge carriers accumulated in a base layer also effects a thermal edge relief. This thyristor requiring two gates is unsuitable for power modules.
From DE-C2-3 509 745, a gate-turn-off thyristor is known in which, for compensating for the unequal feed resistances of a point gate contact in a sector in the area closely around the gate supply point, a longer carrier life is set than in a sector in its far area. This neither attempts nor achieves thermal edge relief.
German Offenlegungsschrift 2 041 727 discloses a semiconductor wafer on which, in an annular zone, several radially aligned GTO thyristor segments are arranged which exhibit no cathode metallization at the edge in order to achieve a uniform current distribution over the entire cathode area by means of an increase in resistance at the edge during turn-off.
In DE-C2-3 722 425, a GTO thyristor having several GTO segments arranged in concentric rings is described in which, for achieving equal turn-off times and to avoid thermal breakdowns, particularly in the thyristor units farther removed from the gate connection, the width of the annular emitter areas at the anode side in a ring farther removed from the gate connection is smaller than the corresponding ring width in a ring located closer to the gate connection.
The disadvantageous factor in this arrangement is that the adaptation of the anode-emitter widths favors not only the turn-off process but also impedes the turn-on process and thus puts the turn-on homogeneity of the element at risk.
From EP-A2-0 283 588, a GTO thyristor is known in which, for uniform activation of the parallel-connected individual elements, GTO thyristor segments arranged in an outer ring exhibit a smaller electrical resistance between cathode contact and gate contact than in an inner ring. In the outer ring, a gate distance from the n-type emitter of 50 .mu.m can be set whilst it is 150 .mu.m in the inner ring. The electrically effective distance can be set by an insulation layer which engages below the respective adjoining metallization of cathode and control electrode. Different resistances can also be achieved by layers with different thicknesses of the p-type base layer at the locations at which it emerges on the surface. At these locations, the p-type base layer can also exhibit a different doping profile; a more highly doped additional layer can also be applied there.
A disadvantage of these methods consists in that the impairment of the unfavorably placed segments depends not only on their position but also on the type of activation. Depending on whether this activation accentuates or suppresses the difference between the segments, the countermeasure initiated according to the arrangement is inadequate or overcompensating.
Gate-turn-off thyristors, so-called GTO thyristors, are used as power semiconductor components, particularly in high-power converters. In these arrangements, the method of pulse-width modulation is frequently used for regulation, in which the GTO thyristors are turned on and off with an approximately constant switching frequency which is independent of input and output frequencies. The electrical power dissipation produced in the elements during this process is an important dimensioning criterion since it actively heats up the component and must therefore be removed by cooling, taking into consideration the maximum permissible transition temperature and the thermal resistances between semiconductor body and heat sink.
The entire electrical power dissipation produced in the semiconductor body can be subdivided into static losses, on the one hand, which depend on the mean pulse cycle and into dynamic losses, on the other hand, which are proportional to frequency and are composed of turn-on and turn-off losses. All these loss components are temperature-dependent. This is why, naturally, the heating-up rate itself also becomes a function of the transition temperature.
Whilst the temperature dependence of most of the loss components (static ones, turn-on) is smaller rather than larger, the turn-off losses per pulse rise distinctly at higher temperatures. Since, at the same time, the storage time also increases which unfavorably influences the current distribution during the turn-off phase, a thermal instability can arise in which some of the element surface is heated up more and more and finally exceeds the permissible maximum temperature without the element as a whole reaching the thermal limit data.
With respect to the relevant prior art, reference is also made to EP-B1-0 200 863 from which a semiconductor component with GTO thyristor and diode structure is also known. The diode structure is mounted on the outer edge of the disk-shaped semiconductor at a distance of .ltoreq.1 mm circularly around GTO thyristor segments annularly arranged and is connected in antiparallel with the GTO thyristor. The charge carrier life in the diode is set to be shorter than in the thyristor by installing heavy metal atoms such as gold or platinum into the silicon base material of the semiconductor component or by electron or gamma irradiation. Between the diode and the GTO gate electrode segments, a resistor or, respectively, a circular protective zone with an electrically insulating passivation layer is provided which decouples the thyristor and diode areas from one another so that only few charge carriers can cross over into the other areas in each case.